The present invention relates to communication systems such as two-way addressable cable television systems and more particularly to a method and apparatus for polling subscriber terminals or the like from a polling center remotely located from comunication centers that service the subscriber terminals.
In communication networks such as cable television systems, it is often necessary or desirable to access information from individual subscriber terminals coupled to the network. In a typical two-way cable television system, each subscriber is provided with an addressable converter that enables the selective reception of authorized television programs. Recent advances in the cable television art have enabled the provision of pay-per-view services, wherein the cable television customer has an opportunity to select specific programs, such as first-run movies, sporting events, and special presentations for viewing at a special fee over and above the standard monthly cable television charges. Other features available on cable television systems include shop at home, bank at home, and viewer opinion polling services.
In order for data to be efficiently transferred from an individual subscriber terminal to the cable system operator's communication center (often referred to as the "headend"), it is desirable for the cable system operator to first rapidly poll all of the subscriber terminals in the cable system to see which terminals have data to report. After completing such a quick poll, the system operator will only have to interrogate those specific terminals (i.e., converters) that indicated they had data to report.
In a more expansive communication network, several individual cable systems can be linked together for polling by a central polling center. One difficulty which has substantially slowed the collection of data from subscriber terminals distributed throughout a multiple system network has been the fact that the time required to retrieve data from the various systems depends upon the particular communication links used to tie the systems back to the polling center. Usually, it will require a greater amount of time to retrieve data from systems that are geographically further away from the polling center than those which are close to the polling center location. In cable television systems, such "linkage delays" are also caused by the various pre-demodulation and post-demodulation filters which are used in sending the cable television signals through the cable. In addition, the various cable systems in multiple system networks are often linked together using telephone lines, and the speed, quality and bandwidth of such telephone lines is a factor in the linkage delays which will occur between the systems and the polling center.
Those skilled in the art will appreciate that the objective of fast polling may be met by using a higher signalling rate or by organizing the subscriber terminal addresses in numerical order corresponding to their geographic location in the network, or both. Such solutions, however, have significant drawbacks. For example, high signalling rate polling requires greater bandwidth than slower polling techniques, and also uses equipment which is considerably more expensive. The alternative, i.e., numerical ordering of subscriber terminal addresses, is impractical for two reasons. First, changes in system topology (such as adding or deleting subscriber terminals) would require large numbers of terminals to be repeatedly relocated to preserve the numerical ordering. Second, subscriber terminal addresses are often held in secrecy because of known deficiencies in electronic security in pay TV control systems.
It would be advantageous to provide a method and apparatus for enabling the quick polling of subscriber terminals, in a communication network that includes various remotely located communication centers, without the disadvantages referred to above. Such a method and apparatus should accommodate various linkage delays between a central polling center and the various remotely located communication centers while maintaining the efficiency of the polling process as high as possible. The present invention relates to such a method and apparatus.